We identified linear B-cell epitopes in the envelope glycoprotein of dengue virus, type 4, strain 814669, by ELISA using 38 overlapping 15 amino acid peptides that span the 493 amino acid sequence of the envelope as predicted from the nucleotide sequence of cloned DNA. Murine poly- and monoclonal antibodies bound a total of nine peptides. Five of these peptides were weakly bound and represented the N-terminus of the envelope. The remaining four, two of which were bound by monoclonals, were located about 60% of the distance from the N- to C-terminus of the molecule, but were not contiguous with each other. Human sera, largely from patients in endemic areas experiencing a secondary infection, identified five regions in the envelope sequence likely to contain linear antigenic sites. These regions spanned the envelope sequence from N- to C-terminus and were defined by the more frequent binding of 20 of the 38 peptides that were nested in contiguous groups of three to five peptides each. The conditions for interpreting the human data are described. Current and future plans involve use of rabbit anti-peptide antibodies and the use of peptides to define murine and T cell epitopes.